The present invention generally pertains to data transfer systems and is particularly directed to a gateway data port for transferring data between asynchronous data transfer systems.
More specifically, the present invention provides a gateway data port that can be interfaced between the type of telemetry system described in U.S. patent application No. 428,545 by Herbert Alan Schwan filed September 1982 for "Telemetry System for Distributed Equipment Control and Equipment Monitors" abandoned, in favor of Ser. No. 758,565 filed July 24, 1985 (now U.S. Pat. No. 4,614,944) and peripheral data transfer systems that operate asynchronously therewith. In the system described in the Schwan patent application, data ports are connected in series to a receiver by an input data line for serial transmission of input data to the receiver over the input data line; and data ports are connected in series to a transmitter by an output data line for serial transmission of output data from the transmitter over the output data line. A clock line provides a clock signal to the data ports for common system timing. In such system, the output and/or input data ports are connected by the respective output and/or input data line serially as a chain of shift register stages. A frame of output data from the transmitter is shifted down the output data line with each shift being clocked by a clock pulse, until the beginning of the data frame reaches the last shift register state (data port) in the chain. At that time a frame pulse simultaneously enables all of the shift register stages (data ports) to transfer their instant data from the output data line to their respective data ports. A frame of input data is shifted serially up the input data line to the receiver in the same manner from data ports that are connected to each other in series to function as stages of a shift register. This type of system is referred to herein as a Schwan system.
Improved Schwan systems are described in U.S. patent application No. 561,567 by Anthony E. Diamond filed Dec. 15, 1983 for "Addressable Port Telemetry System" and in U.S. patent application No. 622,984 by Anthony E. Diamond, filed June 21, 1984 for "Addressable-Port, Daisy Chain Telemetry System with Self-Test Capability,", now U.S. Pat. No. 4,617,566. The systems described in the Diamond patent applications include an address circuit at each port for determining when that port is enabled to transfer data signals onto or from the data line independent of the port's physical location on the data line.
U.S. patent application No. 428,545 by Schwan further describes gateway data ports (with reference to FIGS. 15-18 therein) for transferring data between the Schwan system and peripheral data transfer systems that operate asynchronously therewith.
In some peripheral systems the data transfer protocol provides for transmission of character oriented strings of consecutive data words interspersed by breaks, wherein each data word includes a given predetermined number of bits. An example of such a peripheral system is one using the RS-232-C standard for interfacing data communications equipment (DCE) provided by a communications carrier company with data terminal equipment (DTE) provided by data processing hardware manufacturers. According to this standard, each data word has the predetermined given number of bits and the last bit in each word, which is referred to as the stop bit, always is at least plus three volts. A break between data words is indicated by a zero voltage signal that persists for at least the duration of the predetermined given number of bit positions. When peripheral systems utilizing such a data transfer protocol are interfaced with the Schwan system the data words must be coherently transferred to and from the peripheral system and also be coherently transferred over the data lines of the Schwan system, notwithstanding the asynchronous operation of the respective systems.